This is my pet hate here. They should accept what we would actually do in English: just write Janos without the accent. What annoys me is that Duolingo always says "almost correct" ... no, it's completely correct in English.
If someone's name is Gábor, we can write Gabor as well. We don't have to either call him Gábor or "translate" it to Gabriel.
Translating names is just silly. We don't call Pedro Almodóvar "Peter the Round" or "The round rock"? (Almodóvar is originally from Arabic.) If you can't type the accent mark, you just call him Pedro Almodovar. Foreign diacritics are always optional in English.
Rant rant rant. I report this as often as I can.
On most English keyboards, hold down Alt Gr while pressing the letter key, and it will give it an acute accent.
Incidentally, we /do/ sometimes translate names in real life. I work with many Polish people in England, who often accept the English version of their name rather than waste time teaching the subtle pronunication.
If you are mad because you are failed for lack of letters with diacritics then you may want to reconsider learning a language dependent on them.
To give you an example how important they are, in polish "robić komuś łaskę" - to do something for someone in a despising, nearly merciful manner. Robić komuś laskę - to give someone a ❤❤❤❤❤❤❤.
Quit whining and install hungarian keyboard layout and use on-screen one if you're using a computer.